MAX1134/MAX1135
Offsets resulting from synchronous noise (such as the
conversion clock) are canceled by the MAX1134/
MAX1135s’ calibration scheme. However, because the
magnitude of the offset produced by a synchronous
signal depends on the signal’s shape, recalibration
may be appropriate if the shape or relative timing of the
clock or other digital signals change, which can occur
if more than one clock signal or frequency is used.
Distortion
Avoid degrading dynamic performance by choosing an
amplifier with distortion much less than the
MAX1134/MAX1135s’ THD (-90dB) at frequencies of
interest. If the chosen amplifier has insufficient common-
mode rejection, which results in degraded THD perfor-
mance, use the inverting configuration to eliminate errors
from common-mode voltage. Low-temperature-coeffi-
cient resistors reduce linearity errors caused by resis-
tance changes due to self-heating. To reduce linearity
errors due to finite amplifier gain, use an amplifier circuit
with sufficient loop gain at the frequencies of interest.
DC Accuracy
If DC accuracy is important, choose a buffer with an
offset much less than the MAX1134/MAX1135s’ maxi-
mum offset (±6mV), or whose offset can be trimmed
while maintaining good stability over the required tem-
perature range.
Operating Modes and Serial Interfaces
The MAX1134/MAX1135 are fully compatible with
MICROWIRE and SPI/QSPI devices. MICROWIRE and
SPI/QSPI both transmit a byte and receive a byte at the
same time. The simplest software interface requires
only three 8-bit transfers to perform a conversion (one
8-bit transfer to configure the ADC, and two more 8-bit
transfers to clock out the 16-bit conversion result).
Short Acquisition Mode (24 SCLK)
Configure short acquisition by setting M1 = 0 and M0 =
0. In short acquisition mode, the acquisition time is 5
clock cycles. The total period is 24 clock cycles per
conversion.
Long Acquisition Mode (32 SCLK)
Configure long acquisition by setting M1 = 1 and M0 =
1. In long acquisition mode, the acquisition time is 13
clock cycles. The total period is 32 clock cycles per
conversion.
Calibration Mode
A calibration is initiated through the serial interface by set-
ting M1 = 0 and M0 = 1. Calibration can be done in either
internal or external clock mode, though it is desirable that
the part be calibrated in the same mode in which it will be
used to do conversions. The part remains in calibration
mode for approximately 80,000 clock cycles unless the
calibration is aborted. Calibration is halted if RST or
SHDN goes low, or if a valid start condition occurs.
Software Shutdown
A software power-down is initiated by setting M1 = 1
and M0 = 0. After the conversion completes, the part
shuts down. It reawakens upon receiving a new start
bit. Conversions initiated with M1 = 1 and M0 = 0 (shut-
down) use the acquisition mode selected for the previ-
ous conversion.
Shutdown Mode
The MAX1134/MAX1135 may be shut down by pulling
SHDN low or by asserting software shutdown. In addi-
tion to lowering power dissipation to 4.0µW, consider-
able power can be saved by shutting down the
converter for short periods between conversions. There
is no need to perform a calibration after the converter
has been shut down unless the time in shutdown is
long enough that the supply voltage or ambient temper-
ature has changed.
Supplies, Layout, Grounding,
and Bypassing
For best system performance, use separate analog and
digital ground planes. The two ground planes should
be tied together at the MAX1134/MAX1135. Use pin 3
and pin 14 as the primary AGND and DGND, respec-
tively. If the analog and digital supplies come from the
same source, isolate the digital supply from the analog
with a low-value resistor (10Ω).
The MAX1134/MAX1135 are not sensitive to the order
of AV
DD
and DV
DD
sequencing. Either supply can be
present in the absence of the other. Do not apply an
external reference voltage until after both AV
DD
and
DV
DD
are present.
Be sure that digital return currents do not pass through
the analog ground. All return-current paths must be low
impedance. A 5mA current flowing through a PC board
ground trace impedance of only 0.05Ω creates an error
voltage of about 250µV, or about 2LSBs error with a ±4V
full-scale system. The board layout should ensure that
digital and analog signal lines are kept separate. Do not
run analog and digital lines parallel to one another. If you
must cross one with the other, do so at right angles.
The ADC is sensitive to high-frequency noise on the
AV
DD
power supply. Bypass this supply to the analog
ground plane with 0.1µF. If the main supply is not ade-
quately bypassed, add an additional 1µF or 10µF low-
ESR capacitor in parallel with the primary bypass
capacitor.
16-Bit ADCs, 150ksps, 3.3V Single Supply
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